Fix up computation and display of streaming online videos.

This depends on PR #1337
This broke in all of the i18n url changes; fixing here.  Includes functionality (system was throwing) and display (was flashing double display of topics)

Stream videos from online, when they're not available locally, by setting (in `local_settings.py`):
```
    BACKUP_VIDEO_SOURCE = "http://s3.amazonaws.com/KA-youtube-converted/%(youtube_id)s.%(video_format)s/%(youtube_id)s.%(video_format)s"
```

This should just work.  @aronasorman, can you test this?  Will test #1337 at the same time.  Note that you need to put your central and distributed servers on this, run `update_language_packs`, do the download, and then test all.